In ink-jet printing methods, droplets of ink are directly projected onto a recording medium from very fine nozzles and allowed to adhere to the recording medium to form characters or images. The ink-jet printing methods have become rapidly spread because of various advantages such as easiness of full coloration, low cost, capability of using a plain paper as the recording medium, non-contact with printed characters or images, etc.
In recent years, in order to impart good weathering resistance and good water resistance to printed materials, an ink containing a pigment as a colorant has been extensively used. In addition, from the standpoint of improving ejection stability and storage stability of ink, in order to stably compounding a pigment into a water-based ink, the technologies for enclosing the pigment in a polymer using a dispersant capable of modifying a surface of the pigment, etc., have been developed.
On the other hand, it has been required to print characters or images not only on a high-absorptive recording medium such as those recording media conventionally called a plain paper and a copying paper, etc., but also on a recording medium for commercial printing purposes such as a low-water absorbing coated paper such as an offset coated paper or a non-liquid absorbing resin film such as a polyvinyl chloride resin film, a polypropylene resin film and a polyester resin film.
It is known that when characters or images are printed on the low-water absorbing or non-water absorbing recording medium by the ink-jet printing methods, there tend to occur problems such as prolonged drying time owing to slow absorption or no absorption of liquid components, and deterioration in rub fastness in an initial stage of the printing.
Hitherto, when printing characters or images on the low-water absorbing or non-water absorbing substrate, a solvent-based pigment ink using an organic solvent as a dispersing medium and a UV-curing ink have been mainly employed. This is because when using the aforementioned inks, there occurs such a phenomenon that the organic solvent is penetrated into the low-water absorbing or non-water absorbing substrate and causes swelling of the surface of the substrate to thereby fix the pigment in the substrate, or a firm resin coating is formed on the substrate after printing, so that it is possible to improve anti-bleeding properties and rub fastness of these inks.
However, these conventional inks tend to have such a problem that a large amount of the organic solvent is diffused into air upon drying to cause adverse influence on environments, or a radical initiator or a polymerizable monomer used in the UV-curing ink raises a risk concerning safety. For this reason, at a present time, development of a water-based pigment ink having a less burden on working environments and natural environments has proceeded.
On the other hand, almost all of the conventional water-based pigment inks have such a problem that they are incapable of forming clear characters or images having a high image quality on a low-water absorbing or non-water absorbing substrate.
In consequence, to solve these conventional problems, JP 2008-260139A (Patent Literature 1) discloses a recording method using an aqueous ink containing a glycol ether or an alkanediol in a total amount of 5 to 30% by mass and further containing a fixing resin having a surface tension of 20 to 30 mN/m in an amount of 2 to 20% by mass, in which it is possible to obtain a high-quality print on a non-water absorbing recording medium having a thickness of 5 to 70 μm which is capable of maintaining a uniform thickness without causing any defects such as deformation of the recording medium.
Also, JP 2013-76018A (Patent Literature 2) discloses a water-based pigment ink composition containing a pigment, water, an organic solvent, a pigment dispersant, a resin and a silicone-based compound wherein the organic solvent contains at least one specific glycol monoalkyl ester in an amount of 10 to 30% by mass on the basis of a whole amount of the ink composition, and the resin is in the form of a water-dispersible resin having a glass transition temperature of not lower than 70° C., in which the ink composition is used as an ink composition for ink-jet printing which can exhibit good storage stability as well as good rub fastness, and is free from distortion or bleeding of printed characters or images on a non-water absorbing substrate.